1. Field of the Invention
The invention relates generally to electronic devices and processes for forming the same, and more specifically, to electronic devices and processes for printing at least one layer for the electronic devices.
2. Description of the Related Art
Electronic devices, including organic electronic devices, continue to be more extensively used in everyday life. Examples of organic electronic devices include Organic Light-Emitting Diodes (“OLEDs”). A variety of deposition techniques can be used in forming layers used in OLEDs. Techniques for printing layers include ink-jet printing and continuous printing.
Ink-jet printing has been used extensively in the formation of full-color OLED displays due to its ability to dispense precise amounts of liquid. However, ink-jet printers may not be capable of printing the narrowest of lines. Ink-jet printers dispense liquids as drops. A 40 pL drop can be used, but has a diameter of approximately 41 microns. Even when using state-of-the-art ink-jet technology, a 10 pL drop has a diameter of approximately 26 microns. In addition to having a limited ability to print fine lines, a printing head for an ink-jet printer moves at a rate no greater than approximately 0.1 m/s. A typical printing speed is approximately 0.064 m/s. As a result, ink-jet printing is time consuming, leading to limited throughput of devices.
Additionally, ink-jet printers are limited in their ability to print a wide variety of liquid compositions. For example, the solid concentration of a liquid composition is typically in a range of 0.5 to 1.5 weight percent, with viscosities between 5 and 15 centipoise within a printing head. At higher concentrations (e.g., viscosities at 15 centipoise and higher), the nozzle for the ink-jet printer has an increased likelihood of clogging or not flowing properly. At lower solids concentrations, too much volume needs to be dispensed resulting in poor line width control.
Continuous printing is just starting to become used in printing layers for electronic devices. Continuous printing can be performed using a printing head having a nozzle. The diameter of the nozzle can be in a range of approximately 10 to 50 microns. However, when a liquid composition is printed over a planar substrate at substantially ambient conditions (e.g., when the ambient temperature is 20° C.), the liquid composition can laterally spread to a width of 100 microns or more, as seen from a plan view of the substrate, before the viscosity of the liquid composition is high enough to restrict further lateral spreading.
Multi-nozzle printing may also produce fabrication artifacts called stitching defects. FIG. 1 includes an illustration of an array 100 that may correspond to a display or a sensor array. When lines are printed in sets, the lines at the edges of each set have different shapes or thicknesses compared to lines further within the set. The difference causes stitching defects 120 that are similar to seams seen with fabrics when they are sewn together. The stitching defects 120 may be visible at the time of fabrication, when the electronic device including the array 100 is used, or both. For displays in particular, the stitching defects 120 greatly diminish the visual appearance of the display. Alternatively, a complicated driving scheme may be used to reduce effects (i.e., visual appearance) of the stitching defects 120 when the display is in use. In an extreme situation, even the complicated driving scheme may not be able to fully counter the effects of the stitching defects 120.